Broad band antenna



Nov. 30, 1948.

: L. N. BRILLOUIN BROAD BAND ANTENNA Fild April 24, 1945 Patented Nov. 30, 1948 BROAD BAND ANTENNA Leon NpBrillouin, Providence, R. 1., assignor to Federal Telephone and Radio Corporation, Newark, N. J a corporation of Delaware Application April 24, 1943, Serial No. 484,335

2 Claims. (01. est-33.63)

This invention relates to antennas and in particular to broad band antennas for ultra high frequencies.

An object of my invention is to provide a broad band antenna having a terminal impedance of a value approximating the value of the character istic impedance of free space.

Another object of my invention is to provide a radiating system which has a high reactance for a very broad frequency band extending over several harmonics.

Another object of my invention is to provide a broad band antenna system which maintains substantially constant directivity over a broad band of frequencies. H

Another object of my invention is to provide an antenna system which is aperiodic in the sense that it has substantially no upper limiting frequency.

Another object of my invention is to provide a transmission line and antenna system on which there are substantially no reflections due to abrupt variations in impedance.

A further object of my invention is to provide an antenna which radiates substantially solely vertically polarized waves.

Other features and objects of my invention will be best understood and appreciated from the following description together with the accompanying drawings of which:

Fig. 1 is a View in perspective of an embodiment with parts broken away to illustrate the principles of my invention;

Fig. 1A illustrates a modification of Fig. 1;

Fig. 2 is a sectional View taken along line 2---2 of Fig. l; and

Fig. 3 is a perspective view of another antenna illustrating the principle of my invention, and wherein the radiative or receptive characteristics are substantially unidirectional Referring to Fig. l, I:have illustrated the antenna system of my invention as consisting of substantially two hornlilre elements 2 and 4 providing coaxial horn-shaped surfaces of revolution. The space 6 lying between and confined by these surfaces forms a path along which high frequency waves are propagated.

When the antenna system is employed for the transmission of radio waves, the beginning of the path is considered to be at the point 8 or at the smallest point of what I termed the neck of the elements 2 and 4, and as terminating at the points it and i2 representing the rim of the flared portions of the elements. When my antenna is employed as a receptor for radio waves, the direction of the path is considered as reversed or extending from the rims l0 and I2 to the point 8. The antenna is equally efilcient when employed as a wave transmitter or as a wave receptor.

The propagating path 6 formed by the elements 2 and 4 is analogous to a coaxial transmission line, both conductors of which vary in diameter between the ends thereof.

Because of the change in the diameters of the concentric conductors, there isa variation in the surge or characteristic impedance of the line, and the line is so arranged that the value of this impedance varies exponentially, say in the ratio of 10 to 1 for any length of line equal to one wave length. In a line in which the surge impedance varies so gradually, there is never any sharp discontinuity in the value of the surge impedance sufiicient to cause appreciable reflections of the wave being propagated.

At the rims l0 and I2 of the flared portion of my antenna, which is also one end of the propagating path 6, the distance between the rims is preferably made equal to at least one-half Wave length at the operating frequency. The area between the edges of the rims I0, I2 is substantially a multiple of A A being the wave length in free space. A separation of this amount results in there being a relatively small change in impedance as the propagated wave leaves the path 6 and begins its radiation into free space. In other words, the impedance of the propagating path is at its termination preferably made substantially equal to the impedance of free space. i The greater the separation of thetwo rims l0 and I2, the more closely will the surge impedance of the transmission line at this point be equal to the impedance of free space.

Referring to Fig. 2, I haveillustrated the direction of the electric field l3. This field is substantially radial between the inner and outer conductorsof the transmission line. As the concentric line and therefore the propagation path is gradually fanned out, this electric field becomes substantially vertical as illustrated in Fig. 1 by the field lines. It. The type of wave propagation within the transmission line is preferably made to be of the type known as the TEM mode or in other words, the direction of propagation is at right angles to both the electric and the magnetic field. This is also the type of propagation which a radio wave assumes in free space. The higher the frequency employed, the greater will be the spacing in terms of Wave lengths between the rims l0 and 12, with the result that the match between the principal mode of propagation along the path 3 t on the one hand, and free space propagation on the other hand, will be improved as the frequency increases.

It will be seen therefore that there is practicaly no upper limit of frequency at which my antenna will fail to radiate efficiently with a given antenna structure. The upper limit of frequency is determined only .by the size of the concentric line at the point 8 where it is connected to the transmitter or energy translating device [6. If the size of the concentric line at this point is too great, it would be difiicult to persuade the Waves to propagate in the principle of TEM mode only, and there would then likely be other modes of propagation possible. The effects of other modes would tend to distort the directional characteristics of the system.

If the axis of my antenna is made vertical as shown in Fig. 1, the antenna would be non-directional in azimuth .and radiate substantially vertically polarized Waves. However, at the higher 'frequencies, the vertical characteristic would be narrowed, or compressed and at very high frequencies, the .radiation would take place in substantially a horizontal plane with little radiation either. above or below this plane.

'In Fig. 1, I have illustrated the energy translator [6 as connected to the end 8 of the antenna through a coupling impedance l8. This impedance, in accordance with usual practice, should transmission line I9 is connected between the terminal .8 of the antenna .and the energy translator Hi. In accordance with my invention the characteristic impedance of this transmission line should be made substantially equal to the impedance .of the antenna .at the point 8, or, which amounts'to the same thing, equal to the impedance of the'coupling'impedance l8. A-trans mission line employed "in this manner does not upset the impedance match between the translator and the antenna and therefore introduces no reflections of the propagated Wave.

In 'Fig. 3, I have illustrated another :modification of my invention and one which yields a radiation pattern'which is concentrated in one direction, with a vertical electric field and a horizontal magnetic field. A concentric line having an inner conductor '20 and an outer conductor 2| terminates in two flared fan-shaped antenna members 22 and 23 respectively. If the width to of the antenna members is large compared with their spacing d, and theseparameters vary slowly as a function of their-distance from "a point of origin 24, the impedance of the an- "tenna members may be made to vary exponen- 'tially throughout their-length. This impedance 'should vary from the value of the surge impedance of the line at the point 24 to a value approximately the characteristic impedance of "free space at the ends 25 of the antenna members.

It has been determined that for the open end of the antenna to have approximately the characteristic impedance of free space the cross-secltional area of the wave propagating path at the open end of the antenna members should amount to a multiple of x A being the wave length in free space.

The antenna members may be rotated to the horizontal in which case the polarization of the radiated field is horizontal, corresponding to the direction of .the electric .field.

An energy translating .device 26 may be connected to the end of transmission line as shown in Fig. 3. This device may be similar to the translator [6 of Fig. 1, and may be designed either for transmitting or for receiving purposes. The impedance looking into the device should be substantially equal to the characteristic impedance of the transmission line.

While I have described above the principles of my invention in connection with specific embodiments thereof, it is to be clearly understood that the description is made only by Way of example and not as a .limitation on the scope of my invention as set forth in the objects of my invention and the accompanying claims.

I claim:

1. An antenna system having a substantially constant reactance over a frequency band of several harmonics comprising antenna elements providing two coaxial surfaces of revolution, the surface of each element having a flared portion and a tapering neck portion, each flared portion terminating in an outer rim, said rims having substantially equal diameters and being spaced apart a distance of at least one half wave length at the operating frequency, said flared and neck portions so combining that said surfaces define a Wave propagating path having a variable surge impedance the value of which varies less than the ratio of ten to one over a length of said path equal to one wave length at the operating frequency.

2. An antenna system in accordance with claim 1 in combination with an energy translating device having a coupling impedance, said impedance being connected across the neck portions of said surfaces at a point where said coupling impedance is equal to the surge impedance of said propagating path at said point.

LEON N. BRILLOUIN.

"REFERENCES CITED Ihe following references are of record in the file of this patent:

UNITED STATES PATENTS 1 OTHER REFERENCES Biconical Electromagnetic Horns, Proceedings of the I. R. E., vol. 2'7,"No. 12, Dec. 1939, page 769. 

